A variety of phase difference films are used in a liquid crystal display device for enhancing its performance. The phase difference films are laminated so that the slow axis thereof is inclined at a variety of certain angles with respect to the polarization transmission axis of a polarizer in order to sufficiently exert its function, and the laminate is installed in the liquid crystal display device. The inclination angle of the slow axis with respect to the transmission axis of the polarizer is often neither a parallel angle nor a perpendicular angle. The conventional phase difference film is produced by stretching in a lengthwise or crosswise direction. Due to this principle, the angle of the slow axis with respect to the lengthwise direction of the film can not be other than 0° or 90°. The aforementioned laminate thus has been produced by a batch method in which film pieces that have cut out from a long roll at a certain angle are bonded one by one. Addressing to this inconvenience, there have been proposed a variety of methods for producing a lengthy phase difference film wherein the film is obliquely stretched at a desired angle whereby the direction of the slow axis of the film can be flexibly controlled to any direction that is other than 0° nor 90° with respect to the lengthwise direction. Employment of this obliquely stretched film enables bonding of the films not with the conventional batch method but with a roll-to-roll method, which therefore realizes remarkable improvement in productivity.
For example, JP 2002-86554-A (corresponding to U.S. Pat. No. 6,746,633) discloses a method for stretching an optical polymer film wherein a continuously-supplied polymer film is held at both edges thereof with holding means which are driven along the lengthwise direction of the film for applying a tensile strength to the film for achieving stretching, wherein the stretching is performed under the conditions of that a locus L1 of the holding unit on one edge of the polymer film from a substantial holding commencement point to a substantial release point, a locus L2 of the holding unit on the other edge of the polymer film from a substantial holding commencement point to a substantial release point, and a distance W between the two substantial release points satisfy a relationship of |L2−L1|>0.4W, that a supporting property of the polymer is kept, and that a volatilization fraction is 5% or more, and then the volatilization fraction is reduced with being shrunk.
JP 2003-232928-A discloses a method for producing a lengthy optical film which is obtained by stretching a lengthy film composed of a thermoplastic resin and the optical axis (orientation axis) of which is in a direction that is neither parallel nor perpendicular to a taking-up direction of the lengthy film, wherein in a region in which the film is substantially stretched, moving rates of opposed edges of the film in a width direction are equal while moving distances thereof are different, and a pair of fixtures hold both edges of the film in the width direction, at least one of which moves on a rail having a wave shape with respect to a film surface upon being stretched. It is stated in the same publication that this stretching step may be repeated several times, and this stretching step may be performed after preparative stretching in a lengthwise or crosswise direction.
JP 2004-9543-A discloses a structure of a tenter apparatus for oblique stretching. In that disclosure, there is a description of an obliquely stretched film that is stretched so that an angle α made by film clips and a tenter rail margin becomes an orientation angle of the film.
However, with such a deformation that is much like a shearing in an oblique direction with respect to the traveling direction, the phase difference film formed thereby would have a reduced cohesion force in a thickness direction. Such a film tends to have breakage on the surface layer thereof. If such an obliquely-stretched film is bonded as a protection film on one side to a polarizer, the durability of the resulting polarization plate is remarkably reduced. This is because changes in temperature and humidity cause stress in the vicinity of the bonded interface with the polarizer which readily causes breakage of the phase difference film on the surface thereof, which in turn causes delamination of the film off the polarizer. A liquid crystal display device having such a polarization plate tends to have insufficient ability to maintain good display property over the long period of time.
Thus, it is an object of the present invention to realize production of an obliquely stretched film in which reduction in the cohesion force in the thickness direction of the film is suppressed and in which breakage is also suppressed. It is another object of the present invention to provide a polarization plate having excellent durability, and a liquid crystal display device that can maintain good display property for a long period of time.